Can Yellowstone's signature mammal and the region's ranchers just get along?

Zack Waterman and I are postholing through the snow when I notice the unmistakable oversize tracks of a grizzly. Quickly, we amp up our hollers of "HEEEEYYYYYYYY, BEAR!"

We're headed into Yellowstone National Park's once-remote Pelican Valley, a crescent-shaped dale studded with thermal features. More than a century ago, 23 wild bison found sanctuary there after surviving the slaughter that decimated the 50 million Plains bison that once stretched across the country. Standing in a grove of lodgepoles, Waterman and I fall quiet and listen to the wind shaking the pines. We imagine what it might have felt like to stumble upon the last of North America's largest mammal.

"It's pretty profound that where we found the last wild buffalo is also where we have the opportunity to preserve the species," says Waterman, who at the time of our hike was an organizer for the Sierra Club's Greater Yellowstone campaign and is now director of the Club's Idaho Chapter.

While bison are found across the United States, most of them are restricted to bison ranches. (Bison bison—their scientific name—are commonly called American buffalo, but their large shoulder hump and massive head distinguish them from true buffalo.) Only five states with bison populations allow them to move as free-roaming wildlife, like elk and deer. Yellowstone's 3,700 bison have retained a genetic line free of cattle genes. They are our last wild bison.

But "free to roam" doesn't mean "free of risk." Since 1985, 7,189 Yellowstone National Park bison that have stepped across park boundaries have been killed (or "lethally removed," as the lingo goes) by federal and state agencies in an effort to placate local ranchers' fears that bison will infect their livestock with the disease brucellosis (which has been eradicated in livestock but still exists in bison and elk), damage property, and outcompete their cattle for grass.

Recent developments—including large conservation easements and fencing campaigns, new scientific evidence that counters fears of disease transmission, agreements with tribal leaders, and a plan that may open a large new swath of public land to bison year-round—have many bison advocates optimistic that one key state, Montana, may be on the verge of unprecedented compromise. The battle to protect wild bison won't be over—Yellowstone bison are still shot dead on arrival in Idaho, and their movements are strictly regulated in Wyoming—but it's a significant step toward reclaiming a bit more of the "wild" in wildlife.

Our journey to the Pelican Valley, in Wyoming, provides an informal tour of Yellowstone bison management history and the dichotomy between wildlife and livestock. About 50 miles up the road at Mammoth Hot Springs, concrete remnants of old pens can still be found from the days when Yellowstone National Park kept bison on display for visitors, as in a zoo. Just outside the park's massive stone entrance arch, we pass a capture pen where bison that have wandered out of the de facto animal preserve are held before getting shipped off to slaughterhouses if they test positive for brucellosis antibodies.

Bison are "hazed" by officials on horseback across the Madison River in Yellowstone National Park. | Photo by Molly Loomis

At the Yellowstone border (the end of a nondescript dirt road), we survey an expanded zone outside the park where bison are tolerated—and for the first time in nearly a decade, tribal hunters were recently able to exercise their treaty rights to hunt bison exiting the park. (Recent court decisions have allowed bison, long a part of Native culture, to be transferred to reservations in northeast Montana.)

Next, we detour across the road to the Yellowstone Basin Inn, where a BISON SAFE ZONE sign hangs in the window. Newly built pole-and-rail fences protect trees, backyards, and gardens from free-roaming bison.

"The lodge owners say the only time there is a problem is when the Montana Department of Livestock shows up," Waterman explains. "For a lot of politicians and folks who testify in support of keeping bison out of Montana, one of the biggest arguments is private property rights. But the irony is that the department, which represents Montana ranchers, can enter private property without a landowner's consent."

We pass a large conservation easement north of the park that in 2011 nearly quadrupled the amount of space available to bison in the Gardiner Basin to 70,000 acres. Next we come upon the holding pens leased by the U.S. Department of Agriculture's Animal and Plant Health Inspection Services for grazing studies and experiments with GonaCon, an immunocontraceptive used to sterilize bison with brucellosis.

About 18 miles outside the park boundary, we rattle over a cattle guard marking the mouth of Yankee Jim Canyon, in Gallatin National Forest. I don't give it a second thought until Waterman explains that the cattle guard, installed in 2011, stops bison from moving farther north. And therein lies the heart of the controversy.

At the end of each winter, after months battling harsh conditions, Yellowstone bison descend en masse from the park into the Gardiner and Hebgen Basins in search of emerging plants and comfortable calving sites. Lands in the Yellowstone region are divided into three distinct zones: Zone I is where bison are tolerated year-round—that is, anywhere within the park. Bison are tolerated in Zone II November through May 15, at which point they are "hazed"—herded by bison wranglers on horseback and sometimes helicopters—back into the park. Farther out, Zone III is off-limits to bison year-round. Depending on the time of year, bison found in Zone III are either hazed into the park or killed. To accommodate a growing bison population, the Sierra Club and other groups have been working to expand Zone I to include two large swaths of U.S. Forest Service land—closer than ever to cattle and ranchers.

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Inside the main cabin at Buffalo Field Campaign's headquarters in the Hebgen Basin, an altar displays bison talismans, and buffalo-themed art and civil disobedience stickers decorate the walls. The campaign's volunteers have been zealously documenting bison management efforts since the winter of 1996–97, when Montana's Department of Livestock killed approximately 1,100 Yellowstone bison as they exited the park.

I made my first trip up to the Buffalo Field Campaign's headquarters while snow still blanketed the ground. Stephany Seay, a 10-year veteran of the campaign, showed me the lay of the land. While some ranchers are rabidly anti-bison, several of the Hebgen Basin's ranchers and even residential neighborhoods welcome the bison and other animals that congregate in their front yards like oversize garden gnomes. Prominent neon NO TRESPASSING signs hang in many front windows.

With the May 15 deadline a week away, land management agencies have been staging the bison on Horse Butte, the dividing line between safety and slaughter, for one final haze from Zone II back into the park. Unruly animals that resist will be shot. That's good for land managers, but not for a genome that some say is already at risk, considering the bison management plan's target population of 3,000 in the Yellowstone region—a compromise between land agencies and private interests. It doesn't account for future uncertainties like climate change and disease. The Buffalo Field Campaign claims that 5,000 mature adults are needed to maintain a viable population.

At 8:44 a.m., the campaign's volunteer crews perk up as a Gallatin County sheriff vehicle passes with a Department of Livestock trailer in tow. "The whole shebang is moving out," Cindy Rosin, an art teacher from Queens, New York, with magenta braids, says over the radio.

Two volunteers and I pull our bikes from a truck bed and ride two miles to a turnout overlooking the South Fork of the Madison River, where we spot a herd moving through private land belonging to local rancher Pat Povah. We intersect the haze and spend the next three hours trailing 93 bison, five federal and state riders, and a Forest Service law enforcement vehicle. There's a romantic cowboy feel to it. Except that this isn't a cattle drive; these are wild animals being pushed off migratory land back into what has effectively become a preserve for their species.

Like many stock growers, rancher Povah feels there wasn't a problem until Yellowstone changed its management strategy in the 1980s, allowing herd numbers to fluctuate more naturally. As numbers rose, bison began leaving the park on roads groomed for snowmobiles and snowcoaches and through forests opened up by wildfires in 1989.

"It's the park's responsibility to manage the numbers before they leave the park, and that's not happening," Povah says. "The whole burden is dumped on the surrounding states." He cites the large materials and labor costs associated with repairing fencing, trampled rangeland, and testing cattle for brucellosis.